1. Field of the Invention
The present invention relates to new, polyacrylate-based OH functional binder components, a process for their preparation, and their use to prepare coatings.
2. Description of the Prior Art
The advantage of high solids polyacrylate resins is the reduction in emissions of organic compounds, preferably solvents, during application of the lacquer. In order to obtain these high solids lacquer formulations, appropriate polyacrylate resins having low viscosities, i.e., low molecular weights, must be used.
It is known that free radical solution polymerization can be used to prepare low viscosity polymers (e.g. EP-A 408,858, EP-A 398,387, U.S. Pat. No. 4,145,513). The disadvantage in some cases is that the properties of the polymers are adversely affected by the use of considerable amounts of polymerization regulators as well as their secondary products. In particular the thiols used as polymerization regulators may have an extremely unpleasant smell or may even be poisonous. In EP-A 225,808 and EP-A 225,809 .alpha.-olefins, for example 1-octene or 1-decene, which do not have an unpleasant smell, are therefore used as regulators. However, as comparative tests have shown, two-component (2K) polyurethane (PUR) coating compositions (lacquers) based on the polyacrylate resins described in EP-A 225,809 and aliphatic lacquer polyisocyanates have too long a physical drying time for use in automotive repair lacquers (touch-up lacquers) and large vehicle lacquers.
EP-A 635,523, EP-A 638,591 and EP-A 680,977 describe high solids polyacrylate resins that are prepared by bulk polymerization and then diluted with a suitable solvent. In EP-A 635,523 an attempt was made to apply the technique--known from solution polymerization of using peroxides containing tert.-amyl groups to prepare unimodally narrowly distributed, high solids polyacrylate resins--to bulk polymerization. As expected, it was found that polyacrylate resins having a lower viscosity were obtained with peroxides containing tert.-amyl groups than with peroxides containing tert.-butyl groups. The advantages of this bulk polymerization compared to solution polymerization cannot be demonstrated unequivocally due to the lack of appropriate comparative tests. The polyacrylate resins prepared in the examples had viscosities of between 2790 and 9130 mPas at 23.degree. C. at a solids content of 70 wt. %, which means that they are in the medium solids rather than the high solids range and, thus, do not satisfy present day requirements for high solids lacquers.
EP-A 638,591, describes high solids copolymers that have a relatively low viscosity at a high solids content by using (meth)acrylate monomers with sterically hindered (cyclo)alkyl radicals. In the preparation of these copolymers by bulk polymerization a monoepoxide (Cardura E 10 from Shell) is mentioned as reaction medium in all of the described examples. This monoepoxide reacts with the added (meth)acrylic acid during the polymerization and is thereby incorporated into the polyacrylate. Secondary hydroxyl groups are predominantly formed by this reaction, which in addition to the primary hydroxyl groups (hydroxyethyl methacrylate) or secondary hydroxyl groups (hydroxypropyl methacrylate) introduced by hydroxy-functional monomers, are available for reaction with the polyisocyanate crosslinking agent. The copolymers described in EP-A 680,977 are similar to EP-A 638,591, except that the copolymers used in EP-A 680,977 must have an acid value of at least 15 mg KOH/g.
Secondary hydroxyl groups have a significantly lower reactivity than primary hydroxyl groups with respect to polyisocyanates. 2K-PUR coating compositions based on polyacrylate resins containing more than 30% of secondary hydroxyl groups only attain a sufficient crosslinking density and consequently a satisfactory solvent resistance after several days. However, in automotive refinish lacquers and large vehicle lacquers, fast physical drying as well as a rapid chemical crosslinking are absolutely essential so that the vehicle is available for use again as soon as possible.
Accordingly, it is an object of the present invention to provide new hydroxy-functional copolymers by a simple solution polymerization process, which in combination with (cyclo)aliphatic lacquer polyiso-cyanates result in high solids coating compositions that exhibit fast physical drying and also rapid chemical crosslinking. In contrast to coatings prepared from known coating compositions, it is an object to provide high solids coating compositions that can be used to prepare high quality coatings that possess excellent solvent resistance after curing for only a few days so that coated vehicles are available for use as soon as possible.
It has now surprisingly been found that coating compositions based on binder compositions containing certain high solids graft copolymers and polyisocyanates have a very high solids content in the ready-to-use state. These lacquers may be used to prepare coatings that undergo very rapid physical drying and fast chemical crosslinking at; room temperature (23.degree. C.), i.e., they have an excellent solvent resistance after only 1 to 2 days. The extremely good overall property profile of the resulting coatings, such as hardness, elasticity, chemical resistance and weathering resistance, permits their use in vehicle lacquering, preferably in automotive refinish lacquering and large vehicle lacquering. Other applications include their use to provide corrosion protection and as wood and furniture coatings.